CA1122819A

CA1122819A - Precipitation hardenable iron-nickel-chromium alloy having good swelling resistance and low neutron absorbence

Info

Publication number: CA1122819A
Application number: CA323,877A
Authority: CA
Inventors: Michael K. Korenko; Robert C. Gibson; Howard F. Merrick
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1978-06-22
Filing date: 1979-03-21
Publication date: 1982-05-04
Also published as: DE2910581C2; JPH0127139B2; IT1125955B; DE2910581A1; SE7902558L; FR2429265A1; GB2023651B; NL7901497A; JPS5585648A; BE874958A; SE448743B; US4236943A; GB2023651A; IT7941536A0; FR2429265B1

Abstract

47,104 PRECIPITATION HARDENABLE IRON-NICKEL-CHROMIUM
ALLOY HAVING GOOD SWELLING RESISTANCE AND LOW
NEUTRON ABSORBENCE

ABSTRACT OF THE DISCLOSURE
An iron-nickel-chromium age-hardenable alloy suitable for use in fast breeder reactor ducts and cladding which utilizes the gamma-double prime strengthening phase and characterized in having a morphology of the gamma-double prime phase enveloping the gamma-prime phase and delta phase dis-tributed at or near the grain boundaries. The alloy consists essentially of about 40-50% nickel, 7.5-14% chromium, 1.5-4%
niobium, .25-.75% silicon, 1-3% titanium, .1-.5% aluminum, .02-.1% carbon, .002-.015% boron, and the balance iron. Up to 2% manganese and up to .01% magnesium may be added to inhibit trace element effects; up to .1% zirconium may be added to increase radiation swelling resistance; and up to 3% molybdenum may be added to increase strength.

Description

~e~
~ile not li~lted thereto, the present i~vention is particularly adapted ~or use as a ~ast breeder reactor duct and fuel rod cladding alloy. S~ch ~n alloy requires ~trong mechanical properties at high temperatures and at ~he same time must have both swelling resistance under the lnfluence of irradiation and low neutron absorbence. Alloys such as those descrlbed in U.S. Patent No. 3,046,108 to Eiselstein, disclose age-hardenable n~okel-chromium base alloys which have high strength and good ductility over a wide temperature range up to about 1400F. Speci~ically,, the aforesaid pate~t ~` ~
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~ 47,104 discloses a nickel-base alloy having a nominal composition consisting essentially of about 53~ nickel, ~bout 1g%
chromium, about 3~S moIybQenum, about 5~ niobium, about .2/~
silicon, about 2% manganese, about .9% titanium, about .45/0 aluminum, about .04~/0 carbon and the balance essentially iron.
The alloy is characterized in the age-hardened condition by a yield strength (o.2~0 offset) o~ at least 100,000 pounds per s~uare inch at room temperature and by a 100-hour -~upture strength of at least 90,000 p.s.l. at 1200Fo ~n article by R. Cozar and A. Pineu appearing in "Metallurgical Transactions", Vol. 4, Ja~uary 197~, page 47, explains that nickel-base alloys containing titanium and aluminum, such as those descrlbed in U.S. Patent No. ~,046,108 are strengthen by precipitation of a gamma-prime phase. It has also been found that by adjusting the amounts of titanium aluminum and niobium in such alloys, a morphology can be obtained wherein precipitated gamma-prime particles are coated on their six faces with a shell of gamma-double prime pre-cipitate. The resulting microstructure is very stable on prolonged aging and has thermal stability better than that encountered with most alloys described in U.S. Patent No.
3,046,108.
While the mechanical properties at high temperatures of alloys such as those described above are particularly suit-able for use in nuclear applications, they generally contain in excess of 50% nickel and ln excess of 5% niobium, both of which act as neutron absorbers which makes them undesirable for breeder reactor applications. I~ is, therefore, desirable .~

- , , . ., , , ,, ~ . , , 1~2~8~9 L~7 ,104 to employ an alloy whlch has reduced amounts o~ thQse alloying additions; but at the same time, it has been found that alloys containing about 37,~ nickel, for example, will not precipi-tate the gamma-dQuble phase and that the ratio of atomic percent iron-to-n.ic~el must be less than unity to give the requisite mechanical properties. Thus, the known alloy~, while having the re~uisite mechanical properties, are deficient in one or more respects under the influence o~ irradiation as is encountered, for example, in a fast breeder reactor.
~
The present invention resides in the discovery that the nickel and niobium contents can be decreased in an iron-nickel-chromium alloy containing titanium and aluminum to achieve a reduction in neutron absorbence while at the same time retaining the gamma-prime and gamma-double prime phases to achieve high s~rength mechanical properties at elevated temperatures. The alloy also has good swelling resistance in response to irradiation.
Specifically, it has been found that by reducing the aluminum content of such alloys to about 0.3% and increasing the titanium content to about 1.7%, nickel r-educed from about 53~ to about 45% and niobium ~rom about 5% to as li-ttle as 1.7%, thereby reducing neutron absorbence whlle retaining swelling resistance under irradiation, In addition, the chromium content can be decreased ~rom about 19% to 12% or lower with no deleterious ef~ects.
The above and other objects and features o~ the inventlon will become apparent from the ~ollowing detailed ~, ... . . . . . . .

~ 47,104 description of exemplary embodiments of the i.nvention.

The broad range and preferred compositions of the alloy of the invention are listed in the ~ollowlng Table I:
TABLE I
Broad-~ Prefe red-%
Nickel 40 50 43-47 Chromium 7.5-14 8-12 . Niobium 1.5-4 3-3.8 Silicon .25-.75 .3~.4 Zirconium 0-~1 0-.05 Titanium 1-3 1.5 2 Aliminum .1-.5 .2-.3 Carbon .02-.1 .02-.05 Boron .002~.015 .002-.006 Molybdenum 0~2 0-3 Iron Bal. Bal.

In order to derive the optimized alloy of the inventio~, a number of alloys were examined, the composltions of these alloys being listed in ~he following Table II:

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47,104 TABLE II
y Fe Ni Cr Mo Nb Hf Si D31 Bal 37 12 - 2.5 D32 Bal 37 12 - 4.0 D~3 Bal 45 12 - 4.0 _ _ _ _ D66 Bal 45 12 ~.0 - - 0.5 D~1-M-1 Bal 37 12 - 3.0 0.03 0.5 D31-M-2 Bal ~7 12 - 3.0 0.03 0.5 - ~
D~1-M-3 Bal 37 12 - 3.0 0.03 0.5 - -D31-M 4 Bal 37 1~ - 3.0 0.03 0.5 -D31-M-5 Bal 37 12 ~ 3.0 0.03 0.5 D31-M-6 Bal 37 12 - 3.0 - 0.5 - -D31~M-7 Bal 37 12 2.0 4.0 - 0.5 - -D31-M-8 Bal ~7 12 4,5 ~0 - 0~5 D31-M-9 Bal 37 15 3.0 4.0 - 0.5 0.2 0.02 D31-M-10 Bal 45 12 4.0 - 0~5 0.2 0.02 D31-M-11 Bal 45 12 - 4.0 - 0~5 0.2 0.02 D31-M-12 Bal 45 12 - 4.0 - 005 0.2 0.02 D31-M-13 Bal 45 12 2.0 4,0 - ~5 0.2 0~02 D31-M-14 Bal 45 12 2.0 4.0 - 0-5 0.2 0.02 D31-M-15 Bal 45 12 - 3.6 - 0.5 0.2 0.02 D31-M-16 Bal ~7 12 - 4.0 - 1.5 0.2 0.02 D68 Bal 45 12 - 3.6 - 0.35 0.2 0.01 D69 Bal 37 12 - 4.0 - 0.35 0.2 0.01 Alloy Zr Ti Al C B
D31 0.03 1.0 0.2 0.03 0.010 None D32 0.03 2.8 0.8 0.03 0.010 Y'~ n D33 0.03 1.9 0.5 0.03 0.010 Y~, Y~, D66 0.05 2.5 2.5 0.03 o.oo5 yl 30 D31_M_1 0.03 1.9 0.5 0.03 0.01 None D31-M-2 0.03 1.9 0.8 0.03 0.01 N~ne D31-M~3 0.03 1.9 1.3 0.03 0.01 None D31-M-4 0.03 1.9 -1.6 0.03 0~01 None D31-M-5 0.03 1.9 1.9 0.03 0.01 Y~
D31-M-6 0.05 2.5 2.5 0.03 0.005 Y' D31-M-7 0.05 0.8 0.6 O.G3 0.005 Y' D31-M-8 0.05 0.8 0.6 0.03 0.005 Y' D31-M-9 - 1.0 0.4 0.04 0.005 Y' D31-M-10 0.05 1.8 0.8 0.03 0.005 Y'~
40 D31-M-11 0.05 1.8 1.0 0.03 0.005 Y', D~1-M-12 0.05 1.8 1.2 0.03 0.005 Y'~
D31-M-13 0.05 1.8 0.8 0.03 0.005 Y', D31-M-14 0.5 1.8 1.0 0.03 0.005 Y' 9 D31-M-15 0.05 1.7 0,3 0.03 0.005 **
D31-M 16 0.05 2.6 0.8 0.0~ 0.005 **
D68 0.05 1.7 0.3 0.03 0.007 D69 0.05 2.6 0.8 0.0~ 0.005 ~ . _ *Excluding carbides.
**No~ fabricable.
Alloys a~ed in the range of 16-24 hours at about 7600C.

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~ 47,104 Alloy D31, upon examination of its pho-tomicrograph , dld not contain any precipitates because of the increased solubility of titanium and aluminum in this region of phase space.
Likewise1 Alloy D32 did not produce the gamma-double prime phase because of its relatively low nickel and high alum~num contents. Alloy D33, containing 45% nickel and 12~o chromium contalned not only the gamma-prime and gamma-double prime phases but also the undeslrable delta phase~
In the alloy series D31-M-1 through D31-M-6, the base composition was set at 37~ nickel, 3% niobium, and the balance iron in order to pro~ide a limit on the absorption cross section; and ha~nium, silicon and zirconium were added for swelling resistance. The titanium-to-aluminum ratio was varied in the series D31-M-1 through D31-M-6 l~hich w~uld be expected to produce the gamma-prime an~ gamma-double prime phases in the low aluminum alloys and the gamma-prime phase alone in the high aluminum alloys. Table II shows ? however ?
that alloys D31-M-1 through ~1-M-4 did not contain any precipitates at all except carbides. It is believed that this is due to the fact that alloys in this lower chromlum, intermediate nickel ran~e o~ the phase diagram hav~ a ve~y high solubility ~or titanium and aluminum. Alloys D66 and D~l which contained ~0 titanîum plu~ aluminum and no undèsirable phases further substantiated this conclusion.
Alloys D31-M-7 to D31-M 9 were ~hen melted wlth 4%
niobium and increasing additions o~ moly~denum. mis was ~ done on the basis that molybdenum would decrease the solu~
bility o~ the alloy ~or titan1um and aluminum. The presenc~
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~ ~ 47,104 of the gamma-prime phase in these alloys shows tha-t the anticipated role of molybdenum is correct. mese alloys, which have a titanium plus aluminum con-tent of 1,4% produced the gamma-prime phase. On the other hand, it can be seen from Table II that alloy D31-M-4 containing titanium plus aluminum of ~.5% and no molybdenum, does not contain the gamma-prime phase. In Alloy ~1-M-9, the chromium content was increased from the 12yo ~evel. In¢reasi~g chromium works much like molybdenum in reducing the aluminum plus titanium solubilities, but it does not increase the propensity for gamma~double prime formation. mat is, e~en though ~he titanium-to~aluminum ratios are in the correct range, the gamma-double prime phase will not be observed. For this reason, the iron-to-nickel ratio plays ~ role in determining the limits of phase stabllity ~or gamma~double prime precipi-tate. That is, the ratio of iron-to-nickel must be less than unity As we explained abo~e, it 13 desirable, for nuclear reactor fuel rod cladding applications, to utili~e materials having a low neutron absorbence. Both nlckel and niobium ~:
have high neutron absorbence characteristics; and while increasing the nlobium ~rom the 4% value used in Alloys D31-M-7 through D31-M-9 would shi~t the material into ths gamma-double prime range, niobium is three times as bad as nickel as regards neutron absorbence on a weight perc~nt basis.
There~ore, the only alternative is to increa~e the nickel content as is the case in Alloys D~1-M-10 through D31-M-15 in Table II. To these allo~s, manganese and `~

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47,104 magnesium were added to inhibit trace element embrittlement ef~ects; while ~ilicon was set at 0 5% ~or swelling resis-tance. In this ~eries of alloys, the t~tanium-to aluminum ratios were varied over what was again con~idered to be a reasonable range. Phase extraction analysis of these alloys re~ealed the presence of the gamma-prime and delta phases with no gamma-double prime. Those alloys (i.e., D31-M-13 and 14) containing 2% molybdenum had a greater volume fraction o~
the undesirable delta phase. A comparison of Alloys D33 and D31-M-10 reveals only relatively minor di~ferences in composi tion. Primarily, the difference is in the alu~inum content, being 0.5% in Alloy ~33 which contains the gamma-double prime phase and 0.8% in Alloy D31-M-10 which did not contain the gamma-double prime phase. By lowering the aluminum content to 0.3~, the titanium content to 1.7% and the nioblum content to 3.6%, Alloy D68 was derived which ha~ both the gamma-prime and gamma-double prime phases, relatively low neutron absorbence and good swelling resistance. For maximum swelling resistance in D68 type alloys, the silicon content should be maintained near the upper limit of the range, namely 0~75~.
The nominal composition of the alloy o~ the inven-tion is, there~ore, abou~ 45% nickel, about 12% chromium, about 3.6~ niobium, about 035% silicon, about 1.7% titanium, about .3~ aluminum, about .03~ carbon, about .005% boron and the remainder iron, wlth mangane~e, magnegium and zirconium being optional additions.

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~ 47,104 From the foregoing Table II, it will be apparent that the molybdenum content is not crucial to -the existence of the gamma~double prime ph~se since alloys contalning the gamma-double prime phase with no molybdenum have been pro-duced over the 41.5 to 53.8% nickel range. As the molybdenum content is increased, the solid solu-tion ~trengthening incre-ment of molyb~enum increases and the gamma/gamma prime mis-match is altered~ Increaslng molybdenum decreases the solubility of titanium and aluminum, which are the mos~ e~fec~
tive solid solution strengtheners. The lost strength from a reduced level of titanium and aluminum ln solution is greater than the positive strength increment from molyhdenum. Thus, this result, coupled with the r~sults o~ increasing delta formation with increasing molybdenum and of the high neutron absorption cross section of molybdenum, dictates that molybdenum preferably should be kept as low as possible and under 3%.
The aluminum content is the single most sensitive parameter. Aluminum should be kept as low as pos~ible and no greater than 0.5~, the preferred value being .3%. Again, because of its high neutron absorbence, niobium should be kept low, no greater than 4%.
Once the aluminum content is fixed, the relative and absolute ~alues of titanium and niobium are cruclal. me titanium plus aluminum to~niobium ratio of greater than 1 ~ ~
(when expressed in atomic percent) is a necessary condltion ~ -to produce a gamma-prime/gamma-double prime morphology. -Increasing the titanium content promotes the en~elope :

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l~7 ~ 104 structure. Increas.ing titan~um also reduces swelling, decreases the neutron absorption cross section, and strengthens the alloy by the formation o~ additional gamma-double prime, by solid solution strengthening of the gamma and gamma-prime phases, and by mismatch e~fects. ~hen the composition of Alloy D68 is converted to atomic percent 9 -the (Ti ~ A1)/Nb ratio is 1.1 ~ulfilllng the requirements for the desired morphology.
Alloy D31-M~15 in Table II did not take lnto account fabricability and, there~ore, fractured during hot rolling. The only difference between Alloy D31-~-15 and Alloy D68 which might a~fect fabr~cability are the silicon and manganese levels, both of which are lower in Alloy D68.
Therefore, silicon pre~erably should be kept below .4% and magnesium at about .1%l unless maximum swelling resistance is desired in which event the silicon should be increased to the range between 0.60% and 0.75%.
The alloy of the invention, when aged for 2 hours at 800C, plus furnace cooling to 6250C and holding ~or 1~
hours, has a time to rupt~re of about 280 hours at a testing stress of 621 MPa and a time to rupture of about 2.9 hours at a testing stress o~ 724 MPa.
Although the invention has been shown in connection with certain specific embodlments 9 it should be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to sult reauire-ments without departing ~rom the spirlt and scope o~ the invention.

_10-

Claims

47,104 What is claimed is:

1, An iron-nickel-chromium age-hardenable alloy characterized in having a compact morphology of the gamma-double prime phase enveloping the gamma-prime phase and consisting essentially of, by weight, about 40 to 50% nickel, 7.5 to 14% chromium, 1.5 to 4% niobium, .25 to .75% silicon,

1 to 3% titanium, .1 to .5% aluminum, .02 to .1% carbon, .002 to .015% boron and the balance iron.

2. The alloy of claim 1 in which the ration of iron-to-nickel is less than one.

3. The alloy of claim 1 wherein the ration of Ti +
A1 to Nb, when expressed in atomic percent, is greater than one.

4. The alloy of claim 1 wherein silicon is present in the amount of about .75%.

5. An iron-nickel-chromium age-hardenable alloy characterized in having a compact morphology of the gamma-double prime phase enveloping the gamma prime phase consisting essentially of, by weight, about 43 to 47% nickel, 8 to 12%
chromium, 3 to 3.8% niobium, .3 to .4% silicon, 1.5 to 2%
titanium, .2 to .3% aluminum, .02 to .05% carbon, .002 to .006% boron, and the balance essentially all iron.

6. An iron-nickel-chromium age-hardenable alloy characterized in having a compact morphology of the gamma-double prime phase enveloping the gamma-prime phase and 47,104 consisting essentially of, by weight, about 45% nickel, about 12% chromium, about 3.6% niobium, about .35% silicon, about 1.7% titanium, about 3% aluminum, about .03% carbon, about 0.005% boron and the remainder iron.

7. The alloy of claim 6 additionally containing about .2% manganese, about .01% magnesium, and about .05%
zirconium.